This invention relates to an electronic musical instrument and more particularly an improvement of an electronic musical instrument which forms a musical tone by utilizing a frequency modulation system.
A frequency modulation tone synthesis type electronic musical instrument has been known in which an instantaneous amplitude value e(t) of a signal to be frequency-modulated is calculated according to the following equation and the calculated instantaneous amplitude value e(t) is used as a musical tone signal. EQU e(t)=A.multidot.sin (.omega.ct+I.multidot.sin .omega.mt) (1)
where A represents an amplitude information, I represents a modulation index formation, .omega.ct and .omega.mt represent a carrier frequency and modulation frequency informations corresponding to the note pitch of a depressed key, respectively.
In a musical instrument, such as a pipe organ, the harmonic tone construction of a produced musical tone differs in the high tone range and the low tone range, thus producing musical tones of different tone colors.
However, in the electronic musical instrument of the frequency modulation type described above, since the ratio .omega.c/.omega.m of the carrier angular frequency .omega.c to the modulation angular frequency .omega.m (hereinafter called a frequency modulation ratio) was set corresponding to only the type of the tone color, the harmonic tone construction does not differ in the high tone range and the low tone range of the musical tone so that it has been difficult to produce a natural musical tone having different harmonic tone constructions in the high and low tone ranges as in a pipe organ.